A controversial bill that would require all new cars to be fitted with AM radios looks set to become a law in the near future. Yesterday, Senator Edward Markey (D-Mass) revealed that the “AM Radio for Every Vehicle Act” now has the support of 60 US Senators, as well as 246 co-sponsors in the House of Representatives, making its passage an almost sure thing. Should that happen, the National Highway Traffic Safety Administration would be required to ensure that all new cars sold in the US had AM radios at no extra cost.
I doubt the wavelength is a factor there; depending on the circumstances it could be anything from atmospheric waves to something in your car causing intermittent interference.
The Earth’s ionosphere exists in several layers. During the day, solar radiation ionizes gas deeper into the ionosphere causing a layer that doesn’t usefully refract most radio waves; you can reach beyond the horizon on some of the higher HF bands, but down in the MF, you’ve basically got ground wave. At night, without the sun around to cook the atmosphere, that lower level dissipates, revealing a higher ever-present layer, and the geometry is right to refract signals for hundreds or even thousands of miles.
Skywave propagation can be really fun to play with.
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There likely was no “shielding” in a truck of that era, just simply the truck was made of metal as was the chassis of the radio, bolt 'em together and you’ve got a reasonable ground.
But, I do know from experience that there are items on a pickup truck that can produce radio interference especially when worn. A worn distributor is a spark gap transmitter, as I learned when I installed a mobile radio in my S10. The audio on my radio got a lot better after a good service of the ignition system.